Two-axis scanning mirror using piezoelectric drivers and serpentine torsion springs

ABSTRACT

Embodiments of the disclosure provide a scanning mirror assembly. In certain configurations, the scanning mirror assembly may include a two-dimensional micro-electromechanical system (MEMS) scanning mirror, a skeleton on a back surface of the MEMS scanning mirror, a first pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs, and a second pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs. The first pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a first axis, and the second pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a second axis orthogonal to the first axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. application Ser. No. 17/562,799, entitled “Two-Axis Scanning Mirror Using Piezoelectric Drivers,” filed Dec. 27, 2021, the content of which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a two-axis scanning mirror system of an optical sensing system, and more particularly to, a two-axis scanning mirror driven by two pairs of piezoelectric electrodes coupled to the mirror through serpentine torsion springs.

BACKGROUND

Optical sensing systems, e.g., such as LiDAR systems, have been widely used in advanced navigation technologies, such as to aid autonomous driving or to generate high-definition maps. For example, a typical LiDAR system measures the distance to a target by illuminating the target with pulsed laser light beams and measuring the reflected pulses with a sensor. Differences in laser light return times, wavelengths, and/or phases can then be used to construct digital three-dimensional (3D) representations of the target. Because using a narrow laser beam as the incident light can map physical features with very high resolution, a LiDAR system is particularly suitable for applications such as sensing in autonomous driving and high-definition map surveys.

To scan the narrow-laser beam across a broad field-of-view (FOV) in two-dimension (2D), a scanning mirror has to be able to rotate back and forth about two different axes to scan the surrounding environment. Micro-electromechanical system (MEMS) mirror is an essential component in LiDAR scanner due to its ability to steer optical beams very rapidly. The current state-of-the art MEMS-based LiDAR applications usually require biaxial optical scanning of the surrounding environment using actuators integrated in the systems. Conventionally, 2D scanning is implemented by mounting two separate one-axis scanning mirrors on separate actuators to rotate around the respective axes. Rotation about one axis provides a fast sweep of the surrounding environment and the other axis provides a slow sweep to construct a digital 3D image of the far-field. The slow axis is typically implemented by using mechanical actuator (e.g., a galvanometer) and the fast axis can be implemented by a mechanical or solid-state actuator, The galvanometer may be configured to drive the scanning mirror to rotate about one axis (e.g., slow-sweep), and electrostatic drive combs drive the scanning mirror to rotate about the other axis (e.g., fast-sweep). Galvanometers designed for beam steering applications can have frequency responses up to 1 kHz.

However, there are several drawbacks in using two one-axis mirrors to implement a two-axis scanning mirror (also referred to as a “two-dimensional (2D) mirror”) and using a galvanometer to drive the slow sweep in LiDAR systems. For example, the continued demand for further form factor reductions in optical sensing systems may be constrained. Typically, form factor reductions can be achieved by reducing the number and/or size of the elements included in the system. The two one-axis mirrors are typically placed certain distance away from each other to allow the rotation of each mirror and to accommodate the light path, and therefore taking up a certain space in the LiDAR system. Also, as compared with other elements, the galvanometer occupies a disproportionately large area within the system. Due to the mechanism by which it operates, reducing the size of the galvanometer may be difficult if not impossible to achieve. Moreover, galvanometers are expensive and often suffer from mechanical issue related to its moving parts.

Alternatively, two-axis (2D) MEMS mirrors, which are indeed capable of scanning optical beams in two axes, can be adopted to compensate for the lack of dimensionality of 1D mirrors, and thus reducing the need for a second actuator in the system. Depending on the design and driving mechanism, the mechanical performance of MEMS mirrors may vary significantly. So far only electrostatically driven and electromagnetically driven MEMS mirrors have been developed.

There is an unmet need for a 2D mirror design that can drive a MEMS mirror made of primarily single-crystal silicon into oscillation in both the x-axis and y-axis simultaneously with piezoelectric drivers in both axes.

SUMMARY

Embodiments of the disclosure provide a scanning mirror assembly. In certain configurations, the scanning mirror assembly may include a two-dimensional micro-electromechanical system (MEMS) scanning mirror, a skeleton on a back surface of the MEMS scanning mirror, a first pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs, and a second pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs. The first pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a first axis, and the second pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a second axis orthogonal to the first axis.

Embodiments of the disclosure also provide a transmitter for optical sensing systems. In certain configurations, the transmitter may include a light source configured to emit a light beam towards an object. In certain configurations, the scanner may include a two-dimensional micro-electromechanical system (MEMS) scanning mirror, a skeleton on a back surface of the MEMS scanning mirror, a first pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs, and a second pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs. The first pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a first axis, and the second pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a second axis orthogonal to the first axis.

Embodiments of the disclosure further provide a method for operating a scanner of an optical sensing system. In some embodiments, the method includes applying a first potential to a first pair of piezoelectric electrodes to drive a two-dimensional micro-electromechanical system (MEMS) scanning mirror with a skeleton on a back surface to rotate around a first axis. The first pair of piezoelectric electrodes is coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs. The method further includes applying a second potential to a second pair of piezoelectric electrodes to drive the MEMS scanning mirror and the skeleton to independently rotate around a second axis orthogonal to the first axis. The second pair of piezoelectric electrodes is coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an exemplary LiDAR system, according to embodiments of the disclosure.

FIG. 2A illustrates a first perspective view of an exemplary two-axis scanning mirror assembly, according to embodiments of the disclosure.

FIG. 2B illustrates a second perspective view of the exemplary two-axis scanning mirror assembly of FIG. 2A, according to embodiments of the disclosure.

FIG. 2C illustrates an exemplary two-axis scanning mirror assembly that uses single-folded serpentine torsion springs in both axes, according to embodiments of the disclosure.

FIG. 2D illustrates an exemplary two-axis scanning mirror assembly that uses multiple-folded serpentine torsion springs in both axes, according to embodiments of the disclosure.

FIG. 3A and FIG. 3B illustrate an x-axis rotation of an exemplary scanning mirror when a potential is applied to a first pair of piezoelectric electrodes, according to embodiments of the disclosure.

FIG. 4A and FIG. 4B illustrate a y-axis rotation of an exemplary scanning mirror when a potential is applied to a second pair of piezoelectric electrodes, according to embodiments of the disclosure.

FIG. 5A and FIG. 5B illustrate a z-axis movement of an exemplary scanning mirror when potentials are applied to both first and second pairs of piezoelectric electrodes, according to embodiments of the disclosure.

FIG. 6 is a flowchart of an exemplary process for operating a two-axis scanning mirror assembly, according to embodiments of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

LiDAR is an optical sensing technology that enables autonomous vehicles to “see” the surrounding world, creating a virtual model of the environment to facilitate decision-making and navigation. An optical sensor (e.g., LiDAR transmitter and receiver) creates a 3D map of the surrounding environment using laser beams and time-of-flight (ToF) distance measurements. ToF, which is one of LiDAR's operational principles, provides distance information by measuring the travel time of a collimated laser beam to reflect off an object and return to the sensor. Reflected light signals are measured and processed at the vehicle to detect, identify, and decide how to interact with or avoid objects.

Due to the challenges imposed by using two one-axis mirrors to implement a two-axis mirror and using a galvanometer to drive the slow-sweep axis, as discussed in the BACKGROUND section above, the present disclosure provides a scanner with a true 2D scanning mirror that rotates around two axes and both axes are driven by piezoelectric drivers coupled with serpentine torsion springs for a more compact structure and a smoother operation. It eliminates the need for a galvanometer by including two piezoelectric drivers to drive the scanning mirror to rotate about the two axes simultaneously but independently. The serpentine torsion springs provide further design flexibility for adjusting the mirror oscillation frequency and the rotation angle range of the mirror, as well as reducing stress in the assembly. More specifically, the scanner of the present disclosure can include a first pair of piezoelectric electrodes coupled to the 2D scanning mirror through a first pair of serpentine torsion springs, driving the mirror to rotate around a first axis. The scanner also includes a second pair of piezoelectric electrodes coupled to the scanning mirror through a second pair of serpentine torsion springs, driving the mirror to rotate around a second axis. By eliminating the need for separate mirrors and the galvanometer, the LiDAR system of the present disclosure may be designed with significant reductions in form factor and cost as compared to conventional systems. At the same time, the scanner of the present disclosure steers a laser beam around two axes so that objects in the surrounding environment may be sensed with the degree of accuracy needed for autonomous driving and high-definition map surveys.

Some exemplary embodiments are described below with reference to a scanner used in LiDAR system(s), but the application of the scanning mirror assembly disclosed by the present disclosure is not limited to the LiDAR system. Rather, one of ordinary skill would understand that the following description, embodiments, and techniques may apply to any type of optical sensing system (e.g., biomedical imaging, 3D scanning, tracking and targeting, free-space optical communications (FSOC), and telecommunications, just to name a few) known in the art without departing from the scope of the present disclosure.

FIG. 1 illustrates a block diagram of an exemplary LiDAR system 100, according to embodiments of the disclosure. LiDAR system 100 may include a transmitter 102 and a receiver 104. Transmitter 102 may emit laser beams along multiple directions. Transmitter 102 may include one or more laser source(s) 106 and a scanner 108. Scanner 108 of the exemplary LiDAR system 100 eliminates the need for a bulky and expensive galvanometer. Instead, scanner 108 includes a piezoelectric actuator with two piezoelectric drivers to rotate a scanning mirror about the two axes. In some exemplary scanning operation, one axis can be used to implement the fast-sweep and the another axis can be used to implement the slow-sweep.

Transmitter 102 can sequentially emit a stream of pulsed laser beams in different directions within a scan range (e.g., a range in angular degrees), as illustrated in FIG. 1 . Light source 106 may be configured to provide a laser beam 107 (also referred to as “native laser beam”) to scanner 108. In some embodiments of the present disclosure, light source 106 may generate a pulsed laser beam in the ultraviolet, visible, or near infrared wavelength range.

In some embodiments of the present disclosure, light source 106 may include a pulsed laser diode (PLD), a vertical-cavity surface-emitting laser (VCSEL), a fiber laser, etc. For example, a PLD may be a semiconductor device similar to a light-emitting diode (LED) in which the laser beam is created at the diode's junction. In some embodiments of the present disclosure, a PLD includes a PIN diode in which the active region is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into the active region from the N and P regions, respectively. Depending on the semiconductor materials, the wavelength of incident laser beam 107 provided by a PLD may be greater than 700 nm, such as 760 nm, 785 nm, 808 nm, 848 nm, 905 nm, 940 nm, 980 nm, 1064 nm, 1083 nm, 1310 nm, 1370 nm, 1480 nm, 1512 nm, 1550 nm, 1625 nm, 1654 nm, 1877 nm, 1940 nm, 2000 nm, etc. It is understood that any suitable laser source may be used as light source 106 for emitting laser beam 107. In certain configurations, a collimating lens may be positioned between light source 106 and scanner 108 and configured to collimate laser beam 107 prior to impinging on the MEMS mirror 110. MEMS mirror 110, at its rotated angle, may deflect the laser beam 107 generated by laser sources 106 to the desired direction, which becomes collimated laser beam 109.

Scanner 108 may be configured to steer a collimated laser beam 109 towards an object 112 (e.g., stationary objects, moving objects, people, animals, trees, fallen branches, debris, metallic objects, non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds and even single molecules, just to name a few) in a direction within a range of scanning angles. In some embodiments consistent with the present disclosure, scanner 108 may include, among others, a micromachined mirror assembly having a 2D scanning mirror, such as a MEMS mirror 110 that is individually rotatable about a first axis and a second axis. In some exemplary scanning applications, the first axis (also referred to herein as “x-axis”) may be associated with the fast-sweep or a high-frequency oscillation, and the second axis (also referred to herein as the “y-axis”) may be associated with the slow-sweep or a low-frequency oscillation.

In some embodiments, at each time point during the scan, scanner 108 may steer light from the light source 106 in a direction within a range of scanning angles by rotating the micromachined mirror assembly concurrently (also referred to herein as “simultaneously”) about the first axis and the second axis. That is, the rotations of the 2D scanning mirror about the x-axis and the y-axis can be independent from each other but can occur simultaneously. The range of scanning angles can be designed based on, among others, the Q-factor of the scanning mirror, the voltages applied to the various drivers, the spring constants, and overall system design, etc.

The micromachined mirror assembly may include various components that enable, among other things, the rotation of the MEMS mirror 110 around different axes. For example, the components may include a 2D scanning mirror (e.g., MEMS mirror 110), and a piezoelectric actuator that includes a first piezoelectric driver configured to rotate the scanning mirror around a first axis and a second piezoelectric driver configured to rotate the scanning mirror around a second axis. In some embodiments, MEMS mirror 110 may have a silicon skeleton attached thereto in order to provide support and reduce dynamic deformation of the mirror during rotation. The first piezoelectric driver includes a first pair of piezoelectric electrodes positioned orthogonally across the first axis and the second piezoelectric driver includes a second pair of piezoelectric electrodes positioned orthogonally across the second axis. In some configurations, the piezoelectric actuator can further include a stage at its geometric center, a first pair of serpentine torsion spring positioned along the second axis to join the first pair of piezoelectric electrodes to the stage, and a second pair of serpentine torsion springs positioned along the first axis to join the second pair of piezoelectric electrodes to the stage. The pairs of serpentine torsion springs can be designed to have appropriate spring constants in order to achieve specific mirror oscillation frequencies in the two axes, the rotation angle ranges in the two axes, and to limit the stress in the mirror assembly.

In some further configurations, the 2D scanning mirror may be a single layer of silicon coated with a reflective metal layer. A silicon skeleton may be on the back surface (the silicon surface) of the 2D scanning mirror to provide support, thus reducing the dynamic deformation that causes divergence of optical beams. A post may protrude from the back surface of the 2D scanning mirror. The post connects the 2D scanning mirror with the piezoelectric actuator, in order to make sufficient space for the mirror to rotate or oscillate during a scanning operation. In certain aspects, one or more of the components of scanner 108 may be formed on a single crystal silicon. For example, the scanning mirror, the first driver, and the second driver, just to name a few, may be formed on a single crystal silicon. Additional details of exemplary scanning mirror assembly are set forth below in connection with FIGS. 2A-2D.

Still referring to FIG. 1 , in some embodiments, receiver 104 may be configured to detect a returned laser beam 111 returned from object 112. The returned laser beam 111 may be in a different direction from laser beam 109. Receiver 104 can collect laser beams returned from object 112 and output electrical signals reflecting the intensity of the returned laser beams. Upon contact, laser light can be reflected by object 112 via backscattering, e.g., such as Raman scattering and fluorescence. As illustrated in FIG. 1 , receiver 104 may include a lens 114 and a photodetector 120. Lens 114 may be configured to collect light from a respective direction in its FOV and converge the laser beam to focus before it is received on photodetector 120. At each time point during the scan, returned laser beam 111 may be collected by lens 114. Returned laser beam 111 may be returned from object 112 and have the same wavelength as laser beam 109.

Photodetector 120 may be configured to detect returned laser beam 111 returned from object 112. In some embodiments, photodetector 120 may convert the laser light (e.g., returned laser beam 111) collected by lens 114 into an electrical signal 119 (e.g., a current or a voltage signal). Electrical signal 119 may be generated when photons are absorbed in a photodiode included in photodetector 120. In some embodiments of the present disclosure, photodetector 120 may include a PIN detector, a PIN detector array, an avalanche photodiode (APD) detector, a APD detector array, a single photon avalanche diode (SPAD) detector, a SPAD detector array, a silicon photo multiplier (SiPM/MPCC) detector, a SiP/MPCC detector array, or the like.

LiDAR system 100 may also include at least one signal processor 124. Signal processor 124 may receive electrical signal 119 generated by photodetector 120. Signal processor 124 may process electrical signal 119 to determine, for example, distance information carried by electrical signal 119. Signal processor 124 may construct a point cloud based on the processed information. Signal processor 124 may include a microprocessor, a microcontroller, a central processing unit (CPU), a graphical processing unit (GPU), a digital signal processor (DSP), or other suitable data processing devices.

FIG. 2A illustrates a first perspective view of an exemplary two-axis scanning mirror assembly 200, according to embodiments of the disclosure. FIG. 2B illustrates a second perspective view of the exemplary two-axis scanning mirror assembly 200 of FIG. 2A, according to embodiments of the disclosure. FIG. 2C illustrates an exemplary two-axis scanning mirror assembly 200A that uses single-folded serpentine torsion springs in both axes, according to embodiments of the disclosure. FIG. 2D illustrates an exemplary two-axis scanning mirror assembly 200B that uses multiple-folded serpentine torsion springs in one axis, according to embodiments of the disclosure.

Referring to FIG. 2A, scanning mirror assembly 200 is a “stack” composed of several components. On the very top of the stack is a mirror 202. Mirror 202 can be a 2D scanning mirror that is rotatable around two orthogonal axes, the x-axis and the y-axis, as labeled in FIG. 2A. The x-axis is also referred to as the first axis and the y-axis is referred to as the second axis throughout the descriptions of FIGS. 2A-2D. While a square mirror is illustrated as an example in FIGS. 2A and 2B, it is contemplated that the shape of the mirror could be customized to, e.g., rectangular, circular, triangular, or polygonal, for different applications.

In some embodiments, mirror 202 can be a MEMS mirror formed with a single layer of silicon coated with a reflective metal layer. When the mirror size is larger (e.g., beyond a certain threshold), its dynamic deformation becomes larger as well. The dynamic deformation causes divergence in the reflected optical beams. Therefore, when the dynamic deformation becomes too large, the reflected optical beams may become too divergent, affecting accuracy of scanning. In some embodiments, to reduce the dynamic deformation, a skeleton 206 may be on the back surface of (also referred to as a “back skeleton”). In some embodiments, skeleton 206 may be a silicon layer. In some embodiments, skeleton 206 is designed to cover a large portion of mirror 202 while having a small mass, in order to provide sufficient rigidity to keep mirror 202 flat while keeping the rotational moment of inertia to a minimum. For example, skeleton 206 may be a network skeleton, e.g., a two-dimensional grid.

In some embodiments, skeleton 206 may be formed through an etching process performed on a silicon layer to remove a majority of the mass, leaving only a network. In some designs, the remaining mass of skeleton 206 may be only about 10% of the original silicon layer. Because the rotational moment of inertia is correlated to the mass, by keeping the mass of skeleton 206 small, the moment of inertia is kept low. While a square skeleton 206 is illustrated as an example in FIGS. 2A and 2B, it is contemplated that the shape of the back skeleton can be the same or different from the shape of the mirror. For example, skeleton 206 could be customized to, e.g., rectangular, circular, triangular, or polygonal, for different applications. Also, skeleton 206 can be of the same or different size as mirror 202, although in some designs, it may be substantially the same size as the mirror.

In some embodiments, skeleton 206 may be mounted to mirror 202 during the manufacturing of the mirror, by e.g., bonding. In some alternative embodiments, a thicker layer of silicon may be used to form mirror 202 and skeleton 206 together through, e.g., a partial etching process, on the back side of the silicon.

At the very bottom of the stack, scanning mirror assembly 200 may further include a piezoelectric actuator 210 configured to drive mirror 202, e.g., to rotate around the two axes (x-axis and y-axis), or to vibrate along a third axis normal to the x-y plane (z-axis, not shown). As shown in both FIG. 2A and FIG. 2B, piezoelectric actuator 210 includes two piezoelectric drivers configured to drive mirror 202 to rotate around the two-axes, respectively. As the two piezoelectric drivers operate independent from each other, the rotations of mirror 202 around the two axes are also independent and individual.

In some embodiments, piezoelectric actuator 210 may be bonded to a ceramic substrate (not shown) to form a scanning mirror assembly package. With the use of a two-axis MEMS mirror and piezoelectric drivers for both axes, the size of the assembly package can be made compact.

In some embodiments, as shown in both FIG. 2A and FIG. 2B the first piezoelectric driver may include a first pair of piezoelectric electrodes 212 and second pair of piezoelectric electrodes 214. The piezoelectric electrodes in the first pair are positioned along the second axis (the y-axis) orthogonally across the first axis (the x-axis). The piezoelectric electrodes in the second pair are positioned along the first axis (the x-axis) orthogonally across the second axis (the y-axis). In some configurations, each piezoelectric electrode in the first pair and the second pair may include a relatively thicker silicon plate (e.g., from 10 μm up to 200 μm) coated with a thin piezoelectric film (e.g., 2 μm nominal), such as a lead zirconate titanate (PZT) film. When the electrodes are formed with the PZT films, the piezoelectric driver may be referred to as a PZT driver.

The first piezoelectric driver and the second piezoelectric driver are configured to drive mirror 202 to rotate around the two axes using a converse piezoelectric effect when voltages are applied to the piezoelectric electrodes. For example, one end of each of the first piezoelectric driver and the second piezoelectric driver is affixed to an anchor, and the other end is coupled to the mirror 202 through a pair of serpentine torsion springs. Accordingly, under a drive voltage, the combination of PZT film and silicon support underneath will result in an up or down motion relative to the anchor and provide a force/torque through the loped torsion springs to rotate the mirror about a particular axis. Piezoelectricity is the property of some materials (e.g., PZT, barium titanate, lead titanate, gallium nitride, zinc oxide, etc.) to develop electric charge on their surface when mechanical stress is exerted on them. An applied electrical field produces a linearly proportional strain in these materials. The electrical response to mechanical stimulation is called the direct piezoelectric effect, and the mechanical response to electrical simulation is called the converse piezoelectric effect, which is the mechanism by which the first and second piezoelectric drivers drive mirror 202 to rotate around the x and y axes.

Each piezoelectric film (e.g., PZT film) in the piezoelectric electrodes has a plurality of interlocking crystal domains that have both positive and negative charges. When voltage is not applied to the piezoelectric films, the piezoelectric film and silicon plate on which the film is formed may remain in a neutral position. When a voltage is applied to a piezoelectric film, an outer electrical field is generated that either stretches or compresses the crystal domains in the piezoelectric film causing mechanical strain. When a voltage is applied across a piezoelectric film, the induced strain may cause a pushing (e.g., stretching) or pulling (e.g., compression) of the film depending on whether the voltage is positive or negative. The strain in piezoelectric films may cause stretching and compression of the comparatively rigid silicon plates on which they are formed.

In some embodiments, as shown in FIG. 2B, piezoelectric actuator 210 has a stage 219 in its geometric center. Stage 219 provides a base for other components to join each other. In some configurations, as shown in FIG. 2A, mirror 202 and piezoelectric actuator 210 are connected by a post 204. Post 204 may be a column with any cross-sectional shape, for example, round, elliptical or rectangular, etc. Post 204 is formed on the back surface (e.g., the surface of skeleton 206) of mirror 202 and protrudes from that surface. For example, post 204 may be formed from one-step or two steps DRIE etch from the back of SOI wafers depending on weather the back skeleton support is needed or not. The forming of post 204 may also be achieved through alternative silicon processing sequences. With post 204 in between, mirror 202 and piezoelectric actuator 210 are spaced for a certain distance away from each other. In some embodiments, the height of post 204 may be set to ensure a threshold distance between mirror 202 and piezoelectric actuator 210, so that mirror 202 can rotate/oscillate about the x-axis and y-axis without touching piezoelectric actuator 210. The threshold distance may be determined according to the desired FOV of the scanner. In some embodiments, the height of post 204 may be 500-1000 μm.

In some embodiments, piezoelectric actuator 210 further includes torsion springs along both axes. For example, as shown in FIG. 2B, first torsion springs 216 may be positioned along the first axis (x-axis) and configured to facilitate rotation around the first axis. As illustrated in FIG. 2B, first torsion springs 216 may include two spring parts each located on one side of stage 219 and connect a respect electrode in first pair of piezoelectric electrodes 212 to stage 219. Similarly, second torsion springs 218 may positioned along the second axis (y-axis) and configured to facilitate rotation around the second axis. As illustrated in FIG. 2B, the second pair of torsion springs 218 may also include two spring parts each located on one side of stage 219 and connect a respect electrode in second pair of piezoelectric electrodes 214 to stage 219. In some embodiments, when the mirror size is larger and a skeleton is used, the mirror assembly typically experiences a larger stress. Accordingly, serpentine torsion springs (also referred to as folded torsion springs) may be used as the first and second torsion springs 216 and 218. Serpentine torsion springs are open-looped springs that are capable of absorbing more stress. For example, the adoption of a serpentine spring significantly reduces stress in springs which may be caused by a mechanical shock. The serpentine spring also has smaller stress when displaced from −6° to +6° (mechanical degree) from its neutral position as compared to the stress from a closed loop design

The first and second torsion springs 216 and 218 may each have a spring constant that is implementation specific. Example spring constants for the torsion springs may be as large as one Newton-meter (N*m). By way of example and not limitation, for a 4×12 mm² mirror, the torsion spring constant may be 0.04 N*m; for a 6×12 mm², the torsion spring constant may be 0.1 N*m; for a 10×15 mm², the torsion spring constant may be up to 1 N*m.

In some embodiments, first torsion springs 216 and second torsion springs 218 may adopt different designs, such as the number of folds and the dimension of each fold within the torsion spring, in order to adjust their spring constants. For some configurations, the designs are predetermined according to various factors for designing the scanning mirror assembly. First factor is the mirror oscillation frequency. The spring constant is linearly proportional to the square of a mirror oscillation frequency. In some configurations, first pair of piezoelectric electrodes 212 can drive mirror 202 to oscillate around the first axis at a first frequency, and second pair of piezoelectric electrodes 214 can drive mirror 202 to oscillate around the second axis at a second frequency. In some applications, the slow-axis and fast-axis scanning of scanner 108 may be implemented by scanning mirror assembly 200. In an embodiment where the x-axis is implemented as the fast axis and the y-axis as the slow axis, the first frequency is set to be higher than the second frequency. For example, the first frequency can be 1 kHz, 5 kHz, 10 kHz, 20 kHz, 100 kHz, etc., and the second frequency can be 5 Hz, 10 Hz, 20 Hz, 50 Hz, 100 Hz, etc. Other frequencies suitable for scanning operation may be used. The pair of torsion springs of the respect axis is designed to support the target oscillation frequency of that axis. For example, for a serpentine torsion spring, the overall dimension of the spring (e.g., length L and width W) can be determined by the dimension of each fold and the number of folds included in the spring. Because the spring constant of the torsion spring is correlated to its overall dimension, the constant can be adjusted by changing these two parameters, especially the number of folds. More specifically, the spring constant is essentially proportional to the cubic power of width W and inversely proportional to the cubic power of length L: ˜(W/L)³.

The second factor is the rotation angle range of the mirror. The rotation angle range is designed according to the desired scanning field-of-view (FOV) of the scanner. For example, mirror 202 may be designed to rotate within a −15° to +15° in each axis. In some other examples, the rotation angle range can be any other suitable ranges such as −5° to +5°, or −10° to +10°. It is also contemplated that the rotation angle ranges of the two axes can be the same or different. The range of scanning angles can be designed based on, among others, the Q-factor (quality factor) of the scanning mirror, the voltages applied to the various drivers, the spring constants of the torsion springs, and overall system design, etc.

The third factor is the stress in the scanning mirror assembly, such as stress in the silicon of mirror 202 or the pairs of piezoelectric electrodes (212 or 214). The design goal is to minimize the stress in order to improve the reliability of the mirror. Serpentine torsion springs are selected in the disclosed designs because their ability to absorb a larger stress in a larger mirror. Furthermore, more folds in the serpentine torsion spring could absorb more stress.

Based on the three factors, the serpentine torsion springs in first and second torsion springs 216 and 218 may contain a single fold or multiple folds. For example, FIG. 2C illustrates two-axis scanning mirror assembly 200A that uses first single-folded serpentine torsion springs 216A and second single-folded serpentine torsion springs 218A in both axes. The enlarged views of the torsion springs show that while both first single-folded serpentine torsion springs 216A and second single-folded serpentine torsion springs 218A contain a single fold, the dimensions (e.g., the width and/or height of each fold) of the folds in the respective torsion springs are different. For example, second single-folded serpentine torsion springs 218A has a wider width dimension, thus a larger spring constant, than first single-folded serpentine torsion springs 218A. However, it is contemplated that first single-folded serpentine torsion springs 216A and second single-folded serpentine torsion springs 218A can have a same dimension.

In another example, FIG. 2D illustrates an exemplary two-axis scanning mirror assembly 200B that uses multiple-folded serpentine torsion springs in both axes. For example, each of first multiple-folded serpentine torsion springs 216B and first multiple-folded serpentine torsion springs 218B may include three folds. With more folds in the spring, torsion springs 216B and 218B therefore have a longer length dimension, and thus a smaller spring constant, than torsion springs 216A and 218A. Although FIG. 2D shows an example with an equal number of folds in the x-axis torsion springs and the y-axis torsion springs, it is contemplated that other configurations may use different numbers of folds in the x-axis torsion springs and the y-axis torsion springs. Furthermore, it is also contemplated that the multiple-folded serpentine torsion springs can include any suitable number of folds, depending on the evaluation of the factors above, and not limited to three, as shown in FIG. 2D merely as an example.

FIG. 2C and FIG. 2D are non-limiting examples of scanning mirror assemblies that have different torsion spring designs. It is contemplated that the torsion springs of the two axes (e.g., first torsion springs 216 and second torsion springs 218) could be any combination of single- and multiple-folded serpentine torsion springs. For example, although not shown, one of first torsion springs 216 and second torsion springs 218 may contain a single fold and the other may contain multiple folds.

The mechanical strain caused by the voltage potential applied to a piezoelectric electrode may be passed onto the respective torsion spring. The torsion spring may be stretched or compressed by the force, which in turn rotate or twist stage 219. For example, first torsion springs 216 may cause stage 219 to rotate about the first axis (the x-axis) and second torsion springs 218 may cause stage 219 to rotate about the second axis (the y-axis).

FIG. 3A and FIG. 3B illustrate an x-axis rotation of an exemplary scanning mirror 320 when a potential is applied to a first pair of piezoelectric electrodes, according to embodiments of the disclosure. Similarly, FIG. 4A and FIG. 4B illustrate a y-axis rotation of an exemplary scanning mirror 320 when a potential is applied to a second pair of piezoelectric electrodes, according to embodiments of the disclosure. FIG. 5A and FIG. 5B illustrate a z-axis movement of an exemplary scanning mirror 320 when potentials are applied to both first and second pairs of piezoelectric electrodes, according to embodiments of the disclosure. FIG. 6 is a flowchart of an exemplary process for operating a two-axis scanning mirror assembly, according to embodiments of the disclosure. As shown in FIG. 6 , process 600 may include steps S602-S606 as described below. It is to be appreciated that some of the steps may be optional, and some of the steps may be performed simultaneously, or in a different order than shown in FIG. 6 . FIGS. 3A-6 will be described together.

In step S602, a first potential is applied to a first pair of piezoelectric electrodes to drive the two-axis scanning mirror with a back skeleton to rotate around a first axis. In the example embodiment illustrated in FIG. 3A, the first pair of piezoelectric electrodes for driving mirror 320 to rotate around the first axis (the x-axis) includes electrode 302 and electrode 304. When voltage potentials are applied to piezoelectric electrodes 302 and 304 (e.g., +15V to electrode 302 and −15V to electrode 304), the positive piezoelectric films (+) may bend electrode 302 upward (e.g., 90 μm up) relative to the anchor and the negative piezoelectric films (−) may bend electrode 304 downward (e.g., 90 μm down) relative to the anchor. Accordingly, a mechanical force is created to stretch/compress the torsion springs connected to the electrodes, which then cause the stage in the center to rotate, about the first axis (the x-axis). As the stage is bonded to mirror 320 (e.g., through a post), mirror 320 may be driven to oscillate around the first axis (the x-axis), as illustrated in FIG. 3B. For example, the rotation of mirror 320 under the 15V voltage potentials applied to electrodes 302 and 304 is about 6.6 degrees. Larger voltage potentials may result in a larger rotation range of mirror 320.

In step S604, a second potential is applied to a second pair of piezoelectric electrodes to drive the two-axis scanning mirror with the back skeleton to rotate around a second axis. Similar to FIG. 3A, in the example embodiment illustrated in FIG. 4A, the second pair of piezoelectric electrodes for driving mirror 320 to rotate around the second axis (the y-axis) includes electrode 402 and electrode 404. When voltage potentials are applied to piezoelectric electrodes 402 and 404 (e.g., −15V to electrode 402 and +15V to electrode 404), the positive piezoelectric films (+) may bend electrode 404 upward (e.g., 90 μm up) relative to the anchor and the negative piezoelectric films (−) may bend electrode 402 downward (e.g., 90 μm down) relative to the anchor. Accordingly, the mechanical force is stretches/compresses the torsion springs connected to these electrodes, which then cause the stage in the center to rotate, about the second axis (the x-axis). Mirror 320 may be driven to oscillate around the second axis (the y-axis), as illustrated in FIG. 4B. Again, the rotation of mirror 320 under the 15V voltage potentials applied to electrodes 302 and 304 is about 6.6 degrees around y-axis. Accordingly, the FOV realized by the operation illustrated in FIG. 3B and FIG. 4B will be 6.6 degrees by 6.6 degrees.

In step S606, a third potential is applied to the first pair of piezoelectric electrodes and the second pair of piezoelectric electrodes to move the two-axis scanning mirror with the skeleton along a third axis orthogonal to both the first axis and the second axis. In the example embodiment illustrated in FIG. 5A, the first pair of piezoelectric electrodes includes electrode 302 and electrode 304, and the second pair of piezoelectric electrodes includes electrode 402 and electrode 404. When voltage potentials of the same polarity are applied to piezoelectric electrodes 302, 304, 402 and 404, the mechanical strains will bend all the electrodes up or down collectively, causing the stage to move up or down. For example, when a voltage potential of +15V is applied to each electrode, the positive piezoelectric films (+) may bend each electrode upward (e.g., 90 μm up), driving the stage to elevate along a third axis (the z-axis) orthogonal to both the first axis (the x-axis) and second axis (the y-axis). Because mirror 320 is bonded to the stage of the piezoelectric actuator through a rigid post, mirror 320 elevates along the z-axis accordingly.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed system and related methods. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed system and related methods.

It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 

What is claimed is:
 1. A scanning mirror assembly, comprising: a two-dimensional micro-electromechanical system (MEMS) scanning mirror; a skeleton on a back surface of the MEMS scanning mirror; a first pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs, wherein the first pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a first axis; and a second pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs, wherein the second pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a second axis orthogonal to the first axis.
 2. The scanning mirror assembly of claim 1, wherein the first pair of serpentine torsion springs join the second pair of serpentine torsion springs at a stage.
 3. The scanning mirror assembly of claim 2, further comprising: a post protruding from the MEMS scanning mirror and configured to bond the MEMS scanning mirror to the stage.
 4. The scanning mirror assembly of claim 1, wherein: the first pair of piezoelectric electrodes is configured to oscillate the MEMS scanning mirror around the first axis at a first frequency, the second pair of piezoelectric electrodes is configured to oscillate the MEMS scanning mirror around the second axis at a second frequency, and the first frequency is higher than the second frequency.
 5. The scanning mirror assembly of claim 4, wherein the first pair of serpentine torsion springs has a first number of folds predetermined at least partially based on the first frequency, and the first pair of serpentine torsion springs has a second number of folds predetermined at least partially based on the second frequency.
 6. The scanning mirror assembly of claim 1, wherein the first pair of serpentine torsion springs and the second pair of serpentine torsion springs each contains a single fold.
 7. The scanning mirror assembly of claim 1, wherein the first pair of serpentine torsion springs or the second pair of serpentine torsion springs contains multiple folds.
 8. The scanning mirror assembly of claim 1, further comprising a ceramic substrate, wherein the first pair of piezoelectric electrodes and the second pair of piezoelectric electrodes are electrically connected to the ceramic substrate.
 9. The scanning mirror assembly of claim 1, wherein the first pair of first electrodes or the second pair of piezoelectric electrodes comprises lead zirconate titanate (PZT) films coated on silicon plates.
 10. The scanning mirror assembly of claim 1, wherein the MEMS scanning mirror comprises a single layer of silicon coated with a reflective metal layer.
 11. The scanning mirror assembly of claim 10, wherein the skeleton is mounted to the single layer of silicon.
 12. The scanning mirror assembly of claim 1, wherein the skeleton is a two-dimensional silicon grid.
 13. A transmitter for an optical sensing system, comprising: a light source configured to emit a light beam; and a scanner for steering the light beam towards an object, the scanner comprising: a two-dimensional micro-electromechanical system (MEMS) scanning mirror; a skeleton on a back surface of the MEMS scanning mirror; a first pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs, wherein the first pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a first axis; and a second pair of piezoelectric electrodes coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs, wherein the second pair of piezoelectric electrodes drives the MEMS scanning mirror and the skeleton to rotate around a second axis orthogonal to the first axis.
 14. The transmitter of claim 13, wherein: the first pair of piezoelectric electrodes is configured to oscillate the MEMS scanning mirror around the first axis at a first frequency, and the second pair of piezoelectric electrodes is configured to oscillate the MEMS scanning mirror around the second axis at a second frequency lower than the first frequency.
 15. The transmitter of claim 14, wherein: the first pair of serpentine torsion springs has a first number of folds predetermined at least partially based on the first frequency, and the second pair of serpentine torsion springs has a second number of folds predetermined at least partially based on the second frequency.
 16. The transmitter of claim 13, wherein the first pair of serpentine torsion springs and the second pair of serpentine torsion springs each contains a single fold.
 17. The transmitter of claim 13, wherein the first pair of serpentine torsion springs or the second pair of serpentine torsion springs contains multiple folds.
 18. The transmitter of claim 13, wherein the MEMS scanning mirror rotates comprises a single layer of silicon coated with a reflective metal layer, wherein the skeleton is a two-dimensional silicon grid mounted to the single layer of silicon.
 19. A method for operating a scanner of an optical sensing system, comprising: applying a first potential to a first pair of piezoelectric electrodes to drive a two-dimensional micro-electromechanical system (MEMS) scanning mirror with a skeleton on a back surface to rotate around a first axis, wherein the first pair of piezoelectric electrodes is coupled to the MEMS scanning mirror through a first pair of serpentine torsion springs; and applying a second potential to a second pair of piezoelectric electrodes to drive the MEMS scanning mirror and the skeleton to independently rotate around a second axis orthogonal to the first axis, wherein the second pair of piezoelectric electrodes is coupled to the MEMS scanning mirror through a second pair of serpentine torsion springs.
 20. The method of claim 19, further comprising: applying a third potential to the first pair of piezoelectric electrodes and the second pair of piezoelectric electrodes to move the MEMS scanning mirror and the skeleton along a third axis orthogonal to both the first axis and the second axis. 